The Collaborative International Dictionary of English v.0.48:

Insurance \In*sur"ance\, n. [From Insure.]
   [1913 Webster]
   1. The act of insuring, or assuring, against loss or damage
      by a contingent event; a contract whereby, for a
      stipulated consideration, called premium, one party
      undertakes to indemnify or guarantee another against loss
      by certain specified risks. Cf. Assurance, n., 6.
      [1913 Webster]

   Note: The person who undertakes to pay in case of loss is
         termed the insurer; the danger against which he
         undertakes, the risk; the person protected, the
         insured; the sum which he pays for the protection, the
         premium; and the contract itself, when reduced to form,
         the policy. --Johnson's Cyc.
         [1913 Webster]

   2. The premium paid for insuring property or life.
      [1913 Webster]

   3. The sum for which life or property is insured.
      [1913 Webster]

   4. A guaranty, security, or pledge; assurance. [Obs.]
      [1913 Webster]

            The most acceptable insurance of the divine
            protection.                           --Mickle.
      [1913 Webster]

   5. Hence: Any means of assuring against loss; a precaution;
      as, we always use our seat belts as insurance against
      injury.
      [PJC]

   Accident insurance, insurance against pecuniary loss by
      reason of accident to the person.

   Endowment insurance or Endowment assurance, a combination
      of life insurance and investment such that if the person
      upon whose life a risk is taken dies before a certain
      specified time the insurance becomes due at once, and if
      he survives, it becomes due at the time specified. Also
      called whole life insurance.

   Fire insurance. See under Fire.

   Insurance broker, a broker or agent who effects insurance.


   Insurance company, a company or corporation whose business
      it is to insure against loss, damage, or death.

   Insurance policy, a certificate of insurance; the document
      containing the contract made by an insurance company with
      a person whose property or life is insured.

   Life insurance. See under Life.
      [1913 Webster]

WordNet (r) 3.0 (2006):

insurance company
    n 1: a financial institution that sells insurance [syn:
         insurance company, insurance firm, insurer,
         insurance underwriter, underwriter]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "insurance company":
   accident insurance, actuary, annuity, assurance,
   aviation insurance, bail bond, bond, business life insurance,
   casualty insurance, certificate of insurance, court bond,
   credit insurance, credit life insurance, deductible,
   endowment insurance, family maintenance policy, fidelity bond,
   fidelity insurance, flood insurance, fraternal insurance,
   government insurance, health insurance, industrial life insurance,
   insurance, insurance agent, insurance broker, insurance man,
   insurance policy, interinsurance, liability insurance,
   license bond, limited payment insurance, major medical insurance,
   malpractice insurance, marine insurance, mutual company,
   ocean marine insurance, permit bond, policy, robbery insurance,
   social security, stock company, term insurance, theft insurance,
   underwriter